The period between penetration and replication is the interval in the growth cycle of many viruses which is least understood. Yet, this interval is the stage of the infectious cycle when newly expressed viral gene products first interact with the host cell, thus initiating the chain of events leading to successful infection. These initial interactions between early viral gene products and the host may be critical in determining the course of virus infection which follows. Sindbis virus (SB) causes a lytic infection in vertebrate cells. In cells of invertebrate origin, however, a persistent infection is established in which cultures continue to produce low levels of infectious virus while the cells grow and divide normally. The molecular basis for this dramatic difference in response to infection is unknown. Previous investigations of the early SB non-structural (ns) proteins have focused on lytic infection of baby hamster kidney (BHK) or chick embryo fibroblast (CEF) cells. Virtually nothing is known concerning the synthesis and function of these proteins in persistently infected invertebrate cells. Neither the post-translational modification of the ns proteins nor the effect of SB infection on modification of cellular proteins has been investigated in either lytic or persistent infection. We propose to examine the synthesis, regulation and post-translational modification of the early SB polypeptides during lytic replication in BHK cells and compare the lytic BHK and persistent invertebrate systems with respect to these parameters. Our specific objectives are 1) to determine whether a 49K protein, synthesized soon after SB infection of BHK or CEF cells, is encoded by SB, and to examine the synthesis, regulation and map position of this protein, 2) to explore the possibility that one or more of the SB ns proteins is modified by glycosylation or phosphorylation, and 3) to study the synthesis, modification and control of ns proteins during establishment of persistent infection in Aedes albopictus (AA) cells in comparison to lytic replication of SB in BHK and C6/36 cells, a cloned line of AA which is subject to a cytopathic effect after infection with alphaviruses. We feel that the studies proposed here will elucidate these key aspects of SB replication and may contribute to an understanding of the molecular basis for persistent infections in this and other virus-cell systems.